The present invention is directed to a tin-silver electrolyte. More specifically the present invention is directed to a tin-silver electrolyte that enhances tin-silver alloy deposition on a substrate.
Tin-silver alloy deposition processes have been used in a number of applications requiring attachment of electronic components to printed circuit boards by soldering or reflowing. During assembly, sufficient heat is applied to melt tin-silver alloy deposits and upon cooling, a metallurgical bond between the component and circuit board is formed. Eutectic tin-silver contains 96.5% tin and 3.5% silver, and becomes liquidus at a temperature of 221° C. A number of references disclose electrolytes for depositing silver-tin alloys, including U.S. Pat. No. 5,514,261 and DE patent application 4,330,068.
The electrodeposition of tin-rich alloys of tin-silver is difficult given the large difference in reduction potential between the two metals. Furthermore, the preferential reduction of tin is made more difficult by the fact that silver exists in solution as a monovalent ion, whereas tin is either divalent or tetravalent, and thereby requires two or four times the amount of current for reduction to occur, relative to silver. In addition, an appreciable amount of silver is included in solution to allow for the practical operation of the electrolyte on a production scale. Accordingly, there remains a need in the art for electrolytes that enhance the deposition of tin-silver alloys on a substrate.